1. Field of the Invention
The present invention relates to a thermal flow meter and more particularly is concerned with a flow meter which is less susceptible to a flow turbulence and has a sensor unit to facilitate the assembly in a compact size.
2. Description of the Related Art
As to flow meters used to measure the amount of air sucked into vehicle's engines, such air flow meters as a movable vane type, a Karman vortex type, thermal type and the like are known. Conventional thermal type air flow meters had a problem that the size becomes larger so much as it is inadequate to be mounted in an engine compartment because a sensor control electronic circuit is provided outside the body of the flow meter.
However, a thermal flow meter was recently proposed in which a functional section integrating an electronic circuit and a sensor section is disposed within a flow passage of the thermal flow meter in order to reduce the size. In such a conventional thermal flow meter, since it relies on a method that the flow rate or amount in a bypass passage is measured by a comparatively small sensor in order to monitor the overall flow rate through a main passage and the bypass passage, it is necessary to rectify eccentricity, turbulence and others in the air flow upstream of the sensor.
Where the sensor is provided in the main passage in the a conventional thermal flow meter, the pursuit to reduce size is hindered because it is necessary to provide a dedicated flow duct having a rectifying mechanism such as honeycomb or the like. Where the sensor is provided in the bypass passage, on the other hand, there also is a problem that the measurement is susceptible to the influence of eccentricity in the flow from upstream because the entrance of the bypass passage is eccentric or exit of the bypass passage is opened at a part at the vicinity of wall surface.